I wouldn't mind the extra lift.... but there are a few things that need clarification....

You say you want springs only. With 2" lift ( springs ) the standard suspension is fine to just slap in the springs and the matching shocks.

With you currently having a 4" lift, there is a little more work involved. The caster correction bushes will need to be swapped out to either suit 2" lift or standard possibly just swap out the whole assembly that houses the 2 bushes. Your shocks would need to be swapped out for the matching 2" shocks.

The vehicle taking your 4" lift, will have to swap out all of the above with you in return.

The reason I said I only want to swop the springs is because my shocks are new. If you have shocks of the same condition, then I would definitely also swop shocks to match the coil lift. I spoke to Graeme and he said that I would not have to change the caster bushes for the lower lift lower as it uses the same bushes as a 2" lift, so no work needed there.

As far as I know you can only get a 2 inch correction bush as that's the most the inner metal sleeve can be offset in the bush. The OME bushes are offset and the Nissan Patrol rubber bushes are perfectly in the centre for the best possible flex and comfort.
Here is a pic for the land cruiser showing what happens to the caster with the different lifts. The stock patrol castor is 3.30 degrees =- .30.
When you go over a 2 inch lift the fun starts
What's interesting here is that if you get a 2 inch lift and the 2 degree correction bushes your caster will be 1.5 degrees which is still out of spec for the cruiser?
On my patrol with the OME 2 inch lift and OME 2 degree correction bushes my actual caster was around 2.4 degrees as per the wheel alignment printout

What would be the symptoms if the castor was not corrected with a lift?
AFAIK I got a 3" lift in my 4.5. The rear sway bar brackets was never extended, how would I know if castor correction was implemented or not?
What impact would it have on the vehicles driveability/handling or any other factors to consider?

Fanus, the easiest way to determine if castor correction was done is to take a look at the bushes. If they are black, they are standard rubber. If they are some other colour (yellow normally) they are Polyurethane and thus probably correction.

Castor is what is responsible for "self centering". In other words, a car with a reasonable amount of castor tends to go straight down the road, whereas a car with less castor will require more steering input to keep it going straight.

Of course, as with all things castor also has a drawback. More castor means more steering effort.